A distinguishing characteristic of shrink film is its capacity, upon exposure to some level of heat, to shrink or, if restrained, to create shrink tension within the film. When such a shrink film is used in a process to label or wrap a container, and then is subjected to certain temperature, this process causes the film to shrink around the product producing a tight, transparent or opaque, wrapping that conforms to the contour of the article and provides useful functions required of label or packaging materials.
The ability of a film to shrink upon exposure to some level of heat arises from the orientation of the film during manufacture. During film manufacture, the films are usually heated to their orientation temperature range, which varies with the different polymers used for the films, but is usually above room temperature and below the melting temperature of the polymer. The film is then stretched, either sequentially or simultaneously, in the longitudinal or machine direction (MD) and in the cross or transverse direction (TD) to orient it. After being stretched, the film is rapidly cooled, thus freezing the film in its biaxially oriented state. Upon heating, the orientation stresses are relaxed and the film will begin to shrink back to its original, unoriented dimension.
The polyvinyl chloride (PVC), polystyrene, polyester, and polyolefin families of shrink films provide a wide range of physical and performance film characteristics. Film characteristics play an important role in the selection of a particular film and may differ for each type of packaging or labeling application.
Polyolefins have been most successful with applications where moderate to high shrink forces are preferred. Polyolefin films are also used on automatic, high speed shrink wrapping equipment where shrink and sealing temperature ranges are more clearly controlled. Polyolefin films are particularly suitable for this application because polyolefin films tend to be cleaner, leaving fewer deposits and less residue, which extend the life of the equipment as well as reducing equipment maintenance.
The shrink films have been subjected to subsequent processing steps such as printing, metallizing, or laminating in order to fashion labels for use on containers. Typically, the shrink film is applied to the container to be labeled, and an amount of an adhesive is applied to the shrink film in order to produce tight smooth continuous seams. The application of adhesive to the shrink film is done after fabrication of the shrink film. Application of the adhesive must be accomplished with great care in order to preserve the aesthetics of the label produced from the shrink film. The application of the adhesive layer also is an additional step which may require an amount of time to develop green strength of the adhesive at the seam prior to subsequent processing of labels which high speed labeling and packaging operations may not be able to tolerate.